The present disclosure relates to a rod-shaped implant element for application in spine surgery and/or trauma surgery. The present disclosure further relates to a stabilization device with such a rod-shaped implant element.
Several approaches have been made so far to stabilize the spinal column in the case of intervertebral defects while still allowing a certain degree of motion between the adjacent vertebrae.
EP 0 669 109 B1 discloses a stabilization apparatus for stabilizing neighboring thoracic vertebrae, with which a damaged intervertebral disc and the intervertebral joints can be partly relieved from stress. The apparatus comprises two pedicle screws and a strap that is fixed in the receiver members of each pedicle screw by means of a clamping screw and which further contains a support element that is mounted on the strap and is designed as a pressure-resistant body. However, this stabilization apparatus fails to be torsionally stiff and does not produce any guidance stability of the motion segment of the spinal column.
A joint fixation apparatus, for example for a wrist or a knee joint, is disclosed in U.S. Pat. No. 6,162,223. The apparatus comprises in one embodiment a flexible coupler including an elongated coil spring with two clamps at each end and a flexible cable or a flexible rod extending through the flexible coupler and being fixed at the clamps. Owing to its complex and voluminous structure, this known joint fixation apparatus is not suitable for being used on the spinal column.
US 2003/0220643 A1 discloses a stretchable element to be used in an apparatus for preventing full extension between upper and lower vertebral bodies. The stretchable element may be, for example, an elastic cord or a spring.
US 2003/0191470 A1 discloses an implant device having a rod portion with a center section that flexes when the patient bends their spine. The flexible section can be a linear rod having a variable cross-sectional shape or a non-linear rod having a portion bent in a U-like shape to one side of the axis.
Dynamic stabilization devices are particularly desirable for the treatment of only partially degenerated intervertebral discs. The intervertebral disc should be relieved from stress to support its recovery. If the nucleus of the intervertebral disc is still intact, the intervertebral disc is stiff in the axial direction of the spine (the stiffness is about 500 to 1500 N/mm).
With reference to FIGS. 1 to 5, a known spinal stabilization apparatus for the treatment of degenerated intervertebral discs is shown. Two pedicle screws 2, 3 are anchored in vertebrae 20, 30 which are adjacent to the degenerated intervertebral disc 40 having a nucleus 41, which is still intact. The pedicle screws 2, 3 are connected via a rod-shaped implant element 100 having a helical spring-like section 101 and two rigid sections 102, 103 which serve for the connection with the pedicle screws. FIG. 1 shows the neutral position in which the pedicle screws are spaced apart by a distance x, which changes if the patient bends the spine. In the case of flexion as shown in FIG. 2 or extension as shown in FIG. 3, the nucleus 41 moves in the posterior or anterior direction, respectively, causing the distance between the pedicle screws 2, 3 to decrease or increase relative to the distance x in the neutral position. The decrease and increase of the distance between the pedicle screws 2,3 may vary depending on pathological and anatomical characteristics of an individual. The decrease or increase in the distance between the pedicle screws 2,3 may be nearly the same or different. For example, the decrease and increase can be approximately 1.5 mm and approximately 0.5 mm, respectively, compared to the distance x in the neutral position.
Hence, to allow the flexible section of the rod-shaped implant element to be extended or compressed as required, it has to have a low stiffness or high flexibility in the direction of its longitudinal axis 104. However, if the flexible section 101 has a low stiffness in the axial direction, it also has a low stiffness against bending and shearing forces F as shown in FIG. 5 and bends if such forces occur. This problem arises in the case of use of metal springs as well as in the case of using flexible sections made of other elastic materials such as elastomers.
DE 103 48 329 C1 discloses an elongated rod-shaped implant element comprising a first rigid section to be connected to a bone anchoring element and a second rigid section to be connected to a second bone anchoring element and a flexible section extending between the two rigid sections and formed integrally therewith. The rod-shaped implant element comprises a coaxial bore extending through the element and a core accommodated in the bore, the core being fixed such that it can not slide within the bore.
WO 2004/105577 discloses a spine stabilization system having one or more flexible elements having an opening or a slit in the wall. The flexible element consists of first and second rods which are assembled such that the second rod is fit into a longitudinal bore of the first rod, for example by press-fit.
However, these flexible rod-shaped implant elements do not solve the problem that the rod-shaped implant elements are subject to bending or kinking if they are sufficiently flexible in the axial direction.
In view of the above, there is a need for a rod shaped implant and stabilization device that can remedy one or more of the above described problems associated with current rod shaped implants and stabilization devices.